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    <h1><span>Rasterbar Software</span></h1>
    <h2><span>Software developement and consulting</span></h2>
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<table class="docinfo" frame="void" rules="none">
<col class="docinfo-name" />
<col class="docinfo-content" />
<tbody valign="top">
<tr><th class="docinfo-name">Author:</th>
<td>Arvid Norberg, <a class="last reference external" href="mailto:arvid&#64;rasterbar.com">arvid&#64;rasterbar.com</a></td></tr>
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<div class="section" id="mainline-dht-extensions">
<h1>Mainline DHT extensions</h1>
<p>libtorrent implements a few extensions to the Mainline DHT protocol.</p>
<div class="section" id="get-peers-response">
<h2>get_peers response</h2>
<p>libtorrent always responds with <tt class="docutils literal">nodes</tt> to a get_peers request. If it has
peers for the specified info-hash, it will return <tt class="docutils literal">values</tt> as well. This is
because just because some peer announced to us, doesn't mean that we are
among the 8 closest nodes of the info hash. libtorrent also keeps traversing
nodes using get_peers until it has found the 8 closest ones, and then announces
to those nodes.</p>
</div>
<div class="section" id="forward-compatibility">
<h2>forward compatibility</h2>
<p>In order to support future DHT messages, any message which is not recognized
but has either an <tt class="docutils literal">info_hash</tt> or <tt class="docutils literal">target</tt> argument is interpreted as
find node for that target. i.e. it returns nodes. This allows future messages
to be properly forwarded by clients that don't understand them instead of
being blocked.</p>
</div>
<div class="section" id="client-identification">
<h2>client identification</h2>
<p>In each DHT packet, an extra key is inserted named &quot;v&quot;. This is a string
describing the client and version used. This can help alot when debugging
and finding errors in client implementations. The string is encoded as four
characters, two characters describing the client and two characters interpreted
as a binary number describing the client version.</p>
<p>Currently known clients:</p>
<table border="1" class="docutils">
<colgroup>
<col width="65%" />
<col width="35%" />
</colgroup>
<tbody valign="top">
<tr><td>uTorrent</td>
<td><tt class="docutils literal">UT</tt></td>
</tr>
<tr><td>libtorrent</td>
<td><tt class="docutils literal">LT</tt></td>
</tr>
<tr><td>MooPolice</td>
<td><tt class="docutils literal">MP</tt></td>
</tr>
<tr><td>GetRight</td>
<td><tt class="docutils literal">GR</tt></td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="ipv6-support">
<h2>IPv6 support</h2>
<p><strong>This extension is superseeded by</strong> <a class="reference external" href="http://bittorrent.org/beps/bep_0032.html">BEP 32</a>.</p>
<p>The DHT messages that don't support IPv6 are the <tt class="docutils literal">nodes</tt> replies.
They encode all the contacts as 6 bytes packed together in sequence in a
string. The problem is that IPv6 endpoints cannot be encoded as 6 bytes, but
needs 18 bytes. The extension libtorrent applies is to add another key, called
<tt class="docutils literal">nodes2</tt>.</p>
<p><tt class="docutils literal">nodes2</tt> may be present in replies that contains a <tt class="docutils literal">nodes</tt> key. It is encoded
as a list of strings. Each string represents one contact and is encoded as 20
bytes node-id and then a variable length encoded IP address (6 bytes in IPv4 case
and 18 bytes in IPv6 case).</p>
<p>As an optimization, libtorrent does not include the extra key in case there are
only IPv4 nodes present.</p>
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    <span>Copyright &copy; 2005 Rasterbar Software.</span>
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